1. Field of the Invention
This invention relates to a semiconductor device such as a high-withstand voltage diode, IGBT, thyristor, and more particularly to a high-withstand voltage semiconductor device in which the breakdown strength and power loss are improved.
2. Description of the Related Art
In recent years, a high-withstand voltage semiconductor device such as a high-withstand voltage diode, IGBT, thyristor is widely used and the performance thereof is required to be further enhanced as the performance of devices in the power electronics field such as the inverter- or motor-control is further enhanced and the systemization thereof is further advanced.
For example, the high-withstand voltage diode is formed of a high-resistance N.sup.- -type base layer which is formed of N.sup.- -type silicon, an anode electrode formed on one surface of the base layer with a P.sup.+ -type anode layer disposed therebetween, and a cathode electrode formed on the other surface of the base layer with an N.sup.+ -type cathode layer disposed therebetween.
In the case of a high-withstand voltage diode with a blocking voltage of 4500 V class, the impurity concentrations and the dimensions of the respective portions are set such that the impurity concentration of the N.sup.- -type base layer is set to 1.0.times.10.sup.13 to 1.8.times.10.sup.13 /cm.sup.3 and the thickness thereof is set to 450 to 900 .mu.m, and the surface impurity concentration of each of the P.sup.+ -type anode layer and N.sup.+ -type cathode layer is set to 1.0.times.10.sup.19 /cm.sup.3 and the thickness thereof is set to 14 and 70 .mu.m, respectively.
With the above high-withstand voltage diode, an ON-voltage of approx. 2.6 V can be obtained by a current of approx. 100 A/cm.sup.2. The high-withstand voltage characteristic is attained by forming the junction edge portion in a bevel structure.
In the high-withstand voltage diode with the above structure, a large amount of carriers are stored in the N.sup.- -type base layer in a highly injected state. The carrier concentration becomes high in portions near the N.sup.+ -type cathode layer and P.sup.+ -type anode layer from which electrons and holes are injected.
Since a large amount of carriers are thus stored, a large reverse recovery current flows when a reverse bias voltage is applied to set an OFF state. For example, in a case where the above-described device parameters are used, a large reverse recovery current of approx. 100 A/cm.sup.2 flows when the OFF state is set by a reverse bias voltage of 1000 V and current variation rate di/dt=-200 A/cm.sup.2. Therefore, large electric power is consumed by the reverse recovery current and a large amount of heat is generated. This may prevent the high-speed switching.
As a method for improving the reverse recovery characteristic of the high-withstand voltage diode, it is well known in the art that it is effective to lower the surface impurity concentration of the P.sup.+ -type anode layer and reduce the thickness thereof (for example, refer to IEEE TRANSACTIONS OF ELECTRON DEVICES. VOL-23, NO.8 pp. 945-949, 1976, M. Naito et al., "High Current Characteristics of Asymmetrical P-i-N Diodes Having Low Forward Voltage Drops"). This is considered to be because that the carriers in a portion near the junction whose depletion layer extends at the initial time of the reverse recovery process can be suppressed to a small amount by lowering the hole injection efficiency on the P.sup.+ -type anode layer side.
However, if the surface impurity concentration of the P.sup.+ -type anode layer is lowered, it becomes difficult to sufficiently lower the ohmic contact with the anode electrode, thereby degrading ON characteristic. In order to attain a good ohmic contact which is required to be used for large electric power, it is required to set the surface impurity concentration of the P.sup.+ -type anode layer to approx. 1.times.10.sup.19 /cm.sup.3.
Further, if the impurity concentration of the P.sup.+ -type anode layer is lowered and the thickness thereof is reduced, the depletion layer which extends into the P.sup.+ -type anode layer at the time of application of the reverse bias voltage may almost reach the anode electrode, thereby making it impossible to attain a sufficiently high-withstand voltage characteristic.
Thus, the high-withstand voltage diode has a problem that a large reverse recovery current is caused to flow at the OFF time by the carriers stored in the high-resistance N.sup.- -type base layer to generate a great power loss and heat, thereby destroying the device at the worst case. If an attempt is made to improve the reverse recovery characteristic, a problem that the ON and high-withstand voltage characteristics are deteriorated occurs.
On the other hand, in a field in which a withstand voltage of 2000 V class is required, a planar type high-withstand voltage diode in which the end portion of the junction is formed on the substrate surface by use of a thin anode layer (P-type anode layer) is used instead of the conventional high-withstand voltage diode with bevel structure. This is because the high-withstand voltage diode which is used together with an IGBT with MOS structure may be set in the same package together with the IGBT in some cases and it is required to permit the method for connection of the lead wires or terminals and the electrode structure to be commonly used for those of the IGBT. In the planar type structure, a current is concentrated on the junction edge portion (the bent portion of the junction) and the junction edge portion is set into the high electric field/high current density state at the time of change from the ON state to the OFF state (at the reverse recovery time). At this time, there occurs a problem that the remaining carriers in the periphery (rim) portion of the main element are concentrated in a portion near the end portion of the P-type anode layer so as to cause an avalanche current to partially flow, thereby destroying the device. Further, the withstand voltage of the IGBT is further rapidly enhanced and it is strongly required to enhance the reverse recovery characteristic of the planar type high-withstand voltage diode.